SRTM(Shuttle Radar Topogr.)

Project Summary

The Joint Research Center (together with the International Center for Tropical Agriculture, Colombia) provides processed SRTM 90m Digital Elevation Data for the entire world. The data on this website mirrors the data available at http://srtm.csi.cgiar.org/. The SRTM digital elevation data, originally produced by NASA, is a major breakthrough in digital mapping of the world, and provides a major advance in the accessibility of high quality elevation data for large portions of the tropics and other areas of the developing world. The SRTM digital elevation data provided on this mirror has been processed to fill data voids, and to facilitate it's ease of use by a wide group of potential users. This data is provided in an effort to promote the use of geospatial science and applications for sustainable development and resource conservation in the developing world. Please use http://srtm.csi.cgiar.org/ to search on your area of interest.

Digital elevation models (DEM) for the entire globe, covering all of the countries of the world, are available for download on this site. The SRTM 90m DEM's have a resolution of 90m at the equator, and are provided in mosaiced 5 deg x 5 deg tiles for easy download and use. All are produced from a seamless dataset to allow easy mosaicing. These are available in both ArcInfo ASCII and GeoTiff format to facilitate their ease of use in a variety of image processing and GIS applications. Data can be downloaded using a browser or accessed directly from the ftp site. The DEM data on this site are Version 3 of the CSI-SRTM data with included improved clipping of shorelines using the SRTM Water Bodies Data (SWBD), and improved interpolation of voids areas (using high resolution auxiliary data for US, Canada, Mexico, Europe, Australia, New Zealand and GTOPO30 global topographic data for other areas). If you find this digital elevation data useful, please let us know at csi@cgiar.org.

The NASA Shuttle Radar Topographic Mission (SRTM) has provided digital elevation data (DEMs) for over 80% of the globe. This data is currently distributed free of charge by USGS and is available for download from the National Map Seamless Data Distribution System, or the USGS ftp site. The SRTM data is available as 3 arc second (approx. 90m resolution) DEMs. A 1 arc second data product was also produced, but is not available for all countries. The vertical error of the DEM's is reported to be less than 16m. The data currently being distributed by NASA/USGS (finished product) contains "no-data" holes where water or heavy shadow prevented the quantification of elevation. These are generally small holes, which nevertheless render the data less useful, especially in fields of hydrological modeling.

Dr. Andrew Jarvis and Edward Guevara of the CIAT Land Use project, Dr. Hannes Isaak Reuter (JRC-IES) and Dr. Andy Nelson (JRC-IES), have further processed the original NASA DEMs to fill in these no-data voids. This involved the production of vector contours, and the re-interpolation of these derived contours back into a raster DEM. Three additional points have been added over processing in SRTM V2 available from CIAT: (i) the support for auxiliary information, (ii) the use of a void region specific processing over a tile based processing, and use of SWDB V2 water body database. The algorithm can be downloaded from the CSI-CGIAR website or from this website

These interpolated DEM values were then used to fill in the no-data holes within the SRTM data. This was done using Arc/Info and an AML script. The DEM files have been mosaiced into a seamless global coverage, and are available for download as 5 degree x 5 degree tiles, in geographic coordinate system - WGS84 datum. These files are available for download in both Arc-Info ASCII format, and as GeoTiff, for easy use in most GIS and Remote Sensing software applications. In addition, a binary no data mask file is available for download, allowing users to identify the areas within each DEM which has been interpolated.

We kindly ask any users to cite the Elevation data in any published material produced using this data.Please cite the dataset as: Jarvis A., H.I. Reuter, A. Nelson, E. Guevara, 2008, Hole-filled seamless SRTM data V4, International Centre for Tropical Agriculture (CIAT), available from http://srtm.csi.cgiar.org.

Rules of DISTRIBUTION: Users are prohibited from any commercial, non-free resale, or redistribution without explicit written permission from CIAT. Users should acknowledge CIAT as the source used in the creation of any reports, publications, new data sets, derived products, or services resulting from the use of this data set. CIAT also request reprints of any publications and notification of any redistributing efforts.NO WARRANTY OR LIABILITY: CIAT/JRC provides these data without any warranty of any kind whatsoever, either express or implied, including warranties of merchantability and fitness for a particular purpose. CIAT/JRC shall not be liable for incidental, consequential, or special damages arising out of the use of any data.

SRTM: Methodology

The first processing stage involves importing and merging the 1-degree tiles into continuous elevational surfaces in ArcGRID format. The second process fills the no-data holes through an interpolative technique within an Arc/Info AML model:

The original SRTM DEM (finished grade data downloaded from ftp://e0srp01u.ecs.nasa.gov/srtm/version2/SRTM3/ is used to produce contours or points (depending on the interpolation methodology to be used for the void). Processing was made on a void by void basis.

In cases when a higher resolution auxiliary DEM was available, a point coverage is produced of the elevation values at the centre of each cell of the auxiliary DEM within void areas. When no high resolution auxiliary DEM is available, the 30 second SRTM30 DEM is used as an auxiliary for large voids.

For areas with a high resolution auxiliary DEM: The contours and points surrounding the hole and inside the hole are interpolated to produce a hydrologically sound DEM using the TOPOGRID algorithm in Arc/Info. TOPOGRID is based upon the established algorithms of Hutchinson (1988; 1989), designed to use contour data (and stream and point data if available) to produce hydrologically sound DEMs. This process interpolates through the no-data holes, producing a smooth elevational surface where no data was originally found. Drainage enforcement is activated, and the tolerances set at 5 for "tolerance 1", representing the density and accuracy of input topographic data, and a horizontal standard error of 1m and vertical standard error of 0m.

For areas without a high resolution auxiliary DEM: The most appropriate interpolation technique is selected based on void size and landform typology, and applied on the data immediately surrounding the hole, using SRTM30 derived points inside the hole should it be of a certain size or greater. The best interpolations methods can be generalised as: Kriging or Inverse Distance Weighting interpolation for small and medium size voids in relatively flat low-lying areas; Spline interpolation for small and medium sized voids in high altitude and dissected terrain; Triangular Irregular Network or Inverse Distance Weighting interpolation for large voids in very flat areas, and an advanced Spline Method (ANUDEM) for large voids in other terrains.

The interpolated DEM for the no-data regions is then merged with the original DEM to provide continuous elevational surfaces without no-data regions. This entire process is performed for tiles with large overlap with neighbouring tiles, thus ensuring seamless and smooth transitions in topography in large void areas

The resultant seamless dataset is then clipped along coastlines using the Shorelines and Water Bodies Database (SWBD). This dataset is very detailed along shorelines, and contains all small islands. More information about this dataset is available in USGS (2006c).

The method presented here for filling in the no-data holes in the original SRTM release is by no means the only method available. For a complete review of methods for hole-filling in SRTM data, readers are referred to an article produced by the Alpine Mapping Guild, Gamache (2004). Martin Gamache has since produced some detailed analysis of the data offered here by the CSI, concluding that the hole-filling algorithm is quite successful in representing broad scale patterns in topography in data holes. JRC has performed an advanced analysis of various hole filling algorithms using 1200 relocated void areas in different terrain units and for different void sizes. More information in:Reuter et al. (2007). An evaluation of void-filling interpolation methods for SRTM data. International Journal of Geographical Information Science Volume 21, Issue 9, 2007 .

Voids definition: We define a void in the SRTM dataset as an area over the landsurface, which contains nodata and is not a water body. To give an idea, how many areas are affected by the voids, the following figure describes the percentage of void area per one degree tile. Darker areas indicate regions which contain a high number of voids, light gray areas regions without voids. White areas over continents are the regions where no SRTM data have been provided.

The size of the voids changes quite significantly. The figure below shows the frequency distribution of voids by void size. Please not the huge numbers of small voids with each covering only a small area, in contrast to the less numbers of big voids together with a large area coverage.